This invention relates to apparatus and methods for injecting gas into a fluid and, more particularly, to a lance and mixer configuration and related method for injecting air into a slurry.
In some conventional flue gas desulfurization systems, a wet scrubber brings sulfur-bearing gas into contact with a limestone/water slurry. Sulfur in the flue gas reacts with lime or limestone to produce calcium sulfite (CaSO3). A slurry containing calcium sulfite is contained in large tanks. Typically, air is injected into the slurry to oxidize the calcium sulfite into calcium sulfate (CaSO4). The oxidation reaction makes the slurry easier to handle and the byproduct (calcium sulfate is better known as gypsum) has value and can be recovered.
The mass transfer coefficient, which is a constant of proportionality between the rate of oxygen uptake and the oxygen deficit, is a general measure of the efficiency of an aeration device or configuration in the transfer of oxygen or other gas to water or other fluid. Specifically, the mass transfer coefficient, kla, is a constant for a particular device or configuration in the equation:dc/dt=kla(CS−C)where:                dc/dt=rate of change in concentration of the gas in solution        kla=overall mass transfer coefficient        CS=saturation concentration of gas in solution        C=actual concentration of gas in solution        
A conventional system for injecting air into a slurry is disclosed in published UK Patent Application GB 2 164 576A and is illustrated in FIG. 1 (Prior Art). The conventional system includes a tank 100, a generally vertical gas lance 102, and an impeller 104. The air lance 102 and impeller 104 are submerged in a slurry 106. Flow from the impeller, which is schematically illustrated by directional arrows I in FIG. 1, promotes mass transfer between the gas and liquid.
Operation of the impeller creates swirling flow, which forms a low pressure region at the center of the swirl or vortex. Gas exiting lance 102 may flow backwards into the low pressure region toward impeller 104, which flow is schematically illustrated by directional arrow F in FIG. 1. The term “flooding” is used to refer to the condition in which air from lance 102 contacts the blades of impeller 104. During flooding, the mass transfer efficiency of the system greatly diminishes, and the life of the impeller or its drive may be shortened.